1. Field of the Invention
The present invention relates to a loudspeaker system of a type having a horn behind a loudspeaker unit.
2. Description of the Prior Art
As one example of loudspeaker systems capable of enhancing sounds within a low frequency region, there is well known a loudspeaker system of the back-loaded horn type. This prior art loudspeaker system of the back-loaded horn type comprises a loudspeaker unit having a horn disposed behind the loudspeaker unit and has a cavity defined in the connection between the loudspeaker unit and the throat of the horn. At the low frequency region, the mechanical reactance presented by the cavity, that is, the acoustic volume, is high as compared with the mechanical impedance of the throat of the horn and acoustic sounds behind the loudspeaker unit are coupled directly with the horn. With an increase in the frequency, the mechanical reactance of the acoustic volume lowers to a value smaller than the mechanical impedance of the throat of the horn. Accordingly, the acoustic sounds emanating from the horn attenuate and a relatively large portion of the acoustic output emanates from the front of the loudspeaker unit.
As discussed above, when the horn is coupled with the rear of the loudspeaker unit through the cavity such as in the prior art loudspeaker system, the acoustic radiation efficiency at the low frequency region can be improved by the presence of the horn. As is well known to those skilled in the art, in order to maximize the effect of the horn, for a given flaring constant at which the horn is flared, the opening (mouth) of the horn must be increased. (See "Elements of Acoustical Engineering" by Harry F. Olson, 1940, D. Van Nostrand Co., Inc., p.p. 106 to 109.) The increase of the mouth causes the horn to be bulky and, therefore, in a relatively small-size loudspeaker system, it has been a frequent practice to use a horn having a shape generally similar to a straight tube, that is, a generally straight horn. In such a generally straight horn, the flaring constant is small and the resonance tends to occur at a frequency which is determined by the overall length thereof.
The resonance frequency f.sub.n is expressed by the following equation. EQU f.sub.n =(C/4L).times.(2n+1)(n=0, 1, 2 . . . )
wherein L represents the overall length of the horn, C represents the velocity of sound in the air and n is a positive integer including zero.
Accordingly, when n=0, the quarter wavelength and the horn length are substantially equal to each other enough to induce the resonance with the result that acoustically expanded sounds propagate from the mouth of the horn into the free atmosphere. At the frequency at which one-half wavelength is substantially equal to the horn length, however, sounds propagating from the front of the loudspeaker unit into the free atmosphere and sounds emanating from the rear of the loudspeaker unit which eventually propagate from the mouth of the horn in reverse phase relationship are matched in phase with each other because of a phase delay of one-half wavelength and are therefore mixed together to produce a reinforced increased level of the sound pressure. In view of this, even with the horn having a relatively small flaring constant, a high performance loudspeaker system capable of faithfully reproducing the low frequency range could have been accomplished.
With the prior art loudspeaker system described above, however, it has been found that, at the frequency at which one wavelength is equal to the horn length, the sounds propagating from the front of the loudspeaker unit and the sound emanating from the rear of the same loudspeaker which eventually propagate from the mouth of the horn have their phases opposite to each other enough to counteract with each other. In terms of a characteristic curve of the sound pressure versus frequency, this brings about an interference or reduced level of sound pressure.
Moreover, the cavity defined at the connection between the back of the loudspeaker unit and the horn throat, which cavity acts to attenuate medium-to-high frequency sounds which would propagate from the mouth of the horn, must have a relatively great volume in order for the medium-to-high frequency sounds to be completely attenuated and, in practice, with an increase in frequency, the horn tends to undergo the above described operation repeatedly to such an extent as to result in creation of reinforced and interference portions in sound pressure of composite sounds formed by the sound propagating from the front of the loudspeaker unit and those from the mouth of the horn. This brings about reduction in quality of the sounds reproduced from the loudspeaker system as a whole.